The present invention relates to a narrow profile linear recirculating roller bearing for guiding a linear movement of a machine by rolling friction with a reduced resistance to movement. More particularly, the invention concerns a recirculating bearing which can be made from different types of materials which is not over constrained even when subjected to thermal expansion of rollers and the race.
Many machine tools and precision instruments require a bearing component to move linearly along one axis with minimal resistance. These bearings must normally be fully constrained bearing, that is one which is able to withstand a load in all other directions. Fully constrained bearings are usually preloaded whereby the rollers or other bearing contact surfaces are tightly clamped between the roller guide means. Thermal strains due to expansion can significantly alter the bearing's geometry and preload which can lead to reduced bearing performance, accuracy and useful life.
Many machine tool systems use hardened steel linear bearing assemblies which are bolted to cast iron structures. In this type of system however the coefficients of thermal expansion are different resulting in warping of the components at different temperatures. Thus a machine warm up time is necessary to bring the unit to a proper operating temperature in order to have the bearing function properly. Depending on the machine this may take several hours and thus the need for a warm up time is undesireable for those systems not intended to run 24 hours per day.
Other machine tool systems have used hardened steel for bearing components and aluminum for the structural components. Aluminum's thermal conductivity is five times greater than iron alloys which greatly decreases any required warm up time and decreases the potential for the occurrence of hot spots. However, as the thermal expansion coefficient of aluminum is twice that of steel severe warpage problems may result. Aluminum is also desirable as a structural material because of its light weight, high damping, and ease of machining.
An additional alternative approach to avoiding the adverse affects of thermal strains has been to use materials with a low thermal expansion coefficient and 36% nickels (sold under the trade name Invar). These materials have not proven to be satisfactory due to the expense and difficulty of machining. Additionally, these alloys are seldom hard enough for use as the wear surface, unless fluid film bearings are used.
Recirculating roller bearings are known in the art as disclosed in U.S. Pat. No. 4,302,059 to Teramachi. Disclosed therein is a bearing unit which is intended to reduce the need for turning and grinding operations during production. The bearing unit comprises a bearing body having a pair of downward projections at opposite sides thereof. A pair of parallel sectionally right-angled V-grooves are formed on the inclined load-carrying wall surfaces on the inner side of the projections and a pair of sectionally square U-shaped grooves are formed on the non-load-carrying wall surfaces on the outer sides of the bearing body in parallel relation with the V grooves. A track shaft having cooperating V grooves opposite the bearing body receives a plurality of rollers, alternately shifted 90.degree.. Retainers fixed on the projections form endless roller guide tracks in cooperation with the V and U-grooves. This system, however, is effected by thermal expansion. Additionally, because the bearing relies on the diameter of the roller being equal to the length, the load capacity is limited.
Another example of recirculating roller bearings is shown in U.S. Pat. No. 4,396,235 to Teramachi. This arrangement comprises a bearing unit body having a flat non-loading roller groove where the rollers are essentially horizontal. Two inwardly inclined projections with outer non-loading portions and inner loading portions are also provided. The roller grooves are formed with curved end parts to ensure circulation of the rollers. A track shaft cooperates with the inner grooves to receive rollers adapted to circulate through the upper and lower grooves and the loading and non-loading portions of the projections. This arrangement is also incapable of compensating for thermal expansion.
If different materials are used in the design of these bearings, as the system temperature changes, the distance between the surfaces that the roller contact will change. This causes a non-predictable non-linear change in the preload and bearing gap which will cause a non-linear response throughout the rest of the structure. For economic and manufacturing reasons most machine tools have their main structure made from cast iron and the high stress and wear areas made from hardened steel. These two materials have coefficients of expansion differing by as much as 20% depending on the alloy used.